Question

An object of height 12cm is placed before a concave mirror of focal length 20cm so that a real image is formed at a distance of 60 cm from it. Find the position of the object.​

Answer 1

Height of the object, ho=1.2 m.

Focal length of the concave mirror, f=-20 cm.

Image distance , v=-60cm.

Formula:

(1/f)= (1/u)+(1/v)…………….(1)

Equation (1) is applicable to both concave mirror and convex mirror.

In our problem f and v are given and we are required to find u. From equation (1),

(1/u)=(1/f)-(1/v)=-(1/20)+(1/60)=-(1/30) OR

u=-30 cm.

For height of the image we use the following formula:

Magnification m= hi/ho=-v/u.( for concave mirror)

Here, ho=1.2m , v=-60cm and u=-30cm. Therefore ,

hi=ho(-60/30)=(1.2)(-2)=-2.4m. The negative sign indicates inverted image.